The present invention generally relates to spark plugs for ignition of an air/fuel mixture within a main combustion chamber of an internal combustion engine. In particular, this invention relates to a spark plug configured to allow engine cylinder pressure sensing without requiring modifications to the engine and without interfering with the intended performance of the spark plug.
Spark ignition of an air/fuel mixture within a combustion chamber of an internal combustion engine typically involves igniting the air/fuel mixture with an electric spark jumped between an electrode and a ground electrode of a spark plug. This type of plug, conventionally used in spark ignition systems, produces a fixed flame xe2x80x9ckernelxe2x80x9d that relies on engine design to achieve suitable flame propagation within the combustion chamber. An alternative to spark ignition known in the art is torch jet-assisted spark ignition which, as taught by U.S. Pat. No. 3,921,605 to Wyczalek, U.S. Pat. No. 4,924,829 to Cheng et al., U.S. Pat. No. 5,405,280 to Polikarpus et al., and U.S. Pat. No. 5,421,300 to Durling et al., offers several advantages over spark ignition approaches. As the name suggests, torch jet-assisted spark ignition utilizes a jet of burning gases that are propelled into the combustion chamber in order to enhance the burning rate within the combustion chamber by providing increased turbulence as well as presenting a larger flame front area. As a result of a faster burning rate, lower cyclic variation in cylinder pressure is achieved, which enables a higher engine efficiency with a higher compression ratio.
Regardless of the type of spark plug used, much can be learned of their performance and that of the engines in which they are installed by measuring engine cylinder pressure. In the past, pressure transducers have been separately installed in engine cylinder heads, which requires removing the head, drilling and tapping a hole for the transducer, and then reinstalling the head on the engine. In view of the labor required with this approach, pressure-sensing spark plugs have been developed. Plugs of this type generally have larger diameters than standard spark plugs as a result of a pressure transducer, usually of the piezocrystal-type, being mounted externally to the plug body, making their installation impossible in some engines. The passage required to relay cylinder pressure to the transducer is necessarily narrow and long, which promotes distortion of the pressure signal transmitted through the passage due to through-passage resonance. As a result, the pressure profile that reaches the transducer can yield inaccurate output from the transducer.
A further disadvantage with prior art pressure-sensing spark plugs is the sensitivity of their electrical output to interference and even destruction from the high voltage spark current that is in close proximity. Misleading results from combustion tests using these plugs can also occur because the spark plug firing tip geometry must often by compromised to accommodate the transducer and its passage, causing the plug to not behave in the engine in the same way that a standard plug would.
From the above, it can be appreciated that an improved cylinder pressure sensing device and method would be desirable. Such a device would preferably be less susceptible to distortion of the pressure profile reaching the transducer, insensitive to interference from the high voltage spark current, and permit the use of standard firing tip geometries.
According to the present invention, there is provided a pressure-sensing ignition device for use in a spark ignition system of an internal combustion engine. The ignition device of this invention is configured as a torch jet spark plug, and can therefore serve to ignite an air/fuel mixture within a combustion prechamber within the plug, and then propel the resulting burning gases through an orifice and into the engine main combustion chamber to increase the burning rate of the air/fuel mixture within the combustion chamber. The spark plug has an uncomplicated design and exhibits improved performance by employing the prechamber as a passage to a pressure transducer housed entirely within the body of the plug.
The spark plug of this invention generally includes a body having a longitudinal axis of symmetry, first and second axial ends, and an internal chamber. The chamber has a first axial end and an oppositely-disposed second axial end, with its second axial end being at the second axial end of the body. Disposed at the second axial end of the chamber is an orifice that vents the chamber to the exterior of the body. An electrode and an optical pressure transducer are present within the body. An optical cable is connected to the optical pressure transducer for transmitting an optical pressure signal from the optical pressure transducer. Finally, means associated with the optical cable is provided for conducting a current to the electrode. For example, the optical cable can be provided with a conductor, such as a braided flexible metal sheath, on its exterior surface, which connects to a metallic enclosure on the exterior structure of the optical pressure transducer.
Based on the above construction, the transducer is entirely enclosed within the body of the plug, contrary to prior art pressure-sensing spark plugs that require an externally-mounted transducer. Preferably, the electrode is an annular-shaped center electrode at the first axial end of the chamber, and the optical pressure transducer is in fluidic communication with the chamber through a passage in the center electrode, so that a separate passage dedicated solely to transmitting cylinder pressure to the transducer is unnecessary. Another preferred aspect of the invention is that the optical pressure transducer and electrode are disposed in an axial through-passage within the plug body, a portion of which also defines the chamber.
The ignition device of this invention overcomes the disadvantages of prior art pressure-sensing spark plugs by its use of an internal chamber whose dimensions can be sufficiently generous to be relatively insusceptible to distortion of the pressure profile reaching the transducer. Because the transducer produces an optical signal, another advantage of the invention is that its operation and output are insensitive to interference from the high voltage spark current. Another advantage is that a variety of ground electrodes and firing tip geometries can be employed, including those specifically adapted for torch jet-assisted spark ignition. As a result, the present invention provides an engine cylinder pressure measuring capability to an otherwise standard-sized spark plug, such that the plug can be considered for use in many spark ignition engines. These advantages are not only beneficial to laboratory testing, but also can be employed for pressure-based engine control of a production engine.
Other objects and advantages of this invention will be better appreciated from the following detailed description.